1. Technical Field
The invention relates to inductively coupled plasma reactors for any suitable plasma assisted materials processing, especially semiconductor wafer processing such as plasma etching, chemical vapor deposition, plasma immersion ion implantation and so forth.
2. Background Art
Fabricating multiple-level semiconductor device structures typically requires selectively etching one material faster than another. Also, a vertical etch profile transfer requires anisotropic etching. For example, sub-half micron polysilicon gate etching requires a vertical profile and an etch selectivity ratio of polysilicon over SiO2 of larger than 50 or 100. In a plasma reactor, reactive chemical species can provide very high selectivity, but very poor etch anisotropy. High energy ions can provide good anisotropy but not selectivity. To achieve anisotropic etch with extremely high selectivity, high density ions with very well-controlled energy distribution are required. In recent years, various high density plasma reactors have been developed.
One common feature of these new generation plasma reactors is the independent control of high density ion generation and ion energy control. One kind of reactor of great interest is the inductively coupled plasma reactor. The plasma in such a reactor is generated inside a vacuum chamber by a coiled RF antenna. By adjusting the RF current in the antenna, the ion density can be controlled. The ion energy is controlled by another RF power, usually called RF bias, connected to the wafer pedestal. However, the ion energy is not mono-energetic. The ion energy distribution is dependent on many factors such as bias power and frequency, ion species and plasma density. The capacitive coupling from the antenna to the plasma can cause plasma potential vibration, thereby widening the ion energy distribution. A broad ion energy distribution results in reduction of etch selectivity. However, all state-of-the-art plasma reactors have some capacitive coupling. The capacitive coupling can cause plasma instabilities and undesirable plasma mode changes as well. Moreover, excessive ion bombardment caused by capacitive coupling on the chamber wall adjacent the RF antenna can increase the number of contaminant particles and chamber wear.
In order to reduce such capacitive coupling, a Faraday shield can be placed between the coil antenna and the plasma. The Faraday shield is a grounded thin conductive layer. The Faraday shield, however, must have thin elongate openings therein lying in directions perpendicular to the windings of the RF coil antenna, in order to suppress any eddy currents which would otherwise tend to be induced in the Faraday shield. These openings, however, tend to admit some electric fields from the coil antenna to the plasma, thereby permitting some capacitive coupling.
A related problem is that capacitive coupling from the RF coil antenna prevents gas distribution near the plasma reactor's vacuum chamber ceiling, particularly where the coil antenna is located in the ceiling itself. The ceiling may be a quartz dome, and the RF coil antenna lying in the ceiling is in a dome shape, as disclosed in U.S. patent application Ser. No. 08/113,776, filed Aug. 27, 1993 by Kevin Fairbairn and Romuald Nowak entitled HIGH DENSITY PLASMA CVD AND ETCHING REACTOR and assigned to the present assignee, the disclosure of which is incorporated herein by reference. If gas distribution is too close to the ceiling, capacitive coupling will cause the gas to ionize inside the distribution apparatus and create distribution apparatus wear and contamination throughout the entire reactor from particles generated in the interior of the gas distribution apparatus.
Typically, the RF antenna on the ceiling is operated at 3 to 7 kV, which is sufficient to cause severe ion bombardment of the interior c,f the gas distribution apparatus. For this reason, gas distribution in such a reactor has been confined to regions near the sides of the wafer or the side walls of the vacuum chamber. Such gas distribution (from the sides of the chamber) may cause ion and neutral non-uniform distribution across the wafer surface. Moreover, a great deal of gas so distributed is wasted because both the gas distribution and the vacuum pump aperture are near the chamber sides, so that much of the gas flows directly from the gas distribution ports to the vacuum pump without ever reacting in the plasma. Such problems would be overcome by distributing gas from the entire ceiling of the vacuum chamber. However, there has seemed to be no way of distributing gas from the ceiling due to the possible plasma generation in the gas distribution apparatus by the capacitive coupling from the coil antenna in the ceiling.